A method and device for monitoring dynamic crack propagation based on distributed optical fiber

Abstract

The present application relates to a kind of distributed optical fiber based crack dynamic extension monitoring method and device. Including pre-tilt angle optical fiber according to predetermined spiral winding on the outer surface of cylindrical core, the transformation relationship between the length position of optical fiber and the layer position of core is constructed;In the fracturing process of core, the strain at each length position on the optical fiber corresponding to each time point of fracturing process and the flow of fluid injected in the core are obtained;The flow increment corresponding to each time point and the strain increment at each length position on the optical fiber are calculated respectively;Based on the flow increment corresponding to each time point and the strain increment at each length position, the time point of crack initiation and the length position of crack initiation are determined;According to the transformation relationship, the layer position of crack initiation corresponding to the length position of crack initiation is determined. Through the method of the present application, the measurement accuracy of crack initiation position and crack initiation time is improved.

Description

Technical Field

[0001] The embodiments in this specification relate to the field of shale hydraulic fracturing monitoring technology and rock mechanics experimental methods, and in particular to a method and device for monitoring the dynamic propagation of fractures based on distributed optical fibers. Background Technology

[0002] In addition to the main fracture, numerous secondary fractures (branch fractures) often arise on weak bedding surfaces of shale during hydraulic fracturing of shale reservoirs. The dynamic propagation behavior of these secondary fractures directly affects the complexity of the fracture network and the conductivity of the reservoir. However, current monitoring technologies for fracture propagation in shale hydraulic fracturing have many limitations. Traditional monitoring methods, such as microseismic, acoustic emission, and strain gauge measurements, have limited spatial resolution and are difficult to implement simultaneous multi-point measurements, making it difficult to accurately capture the initiation location, real-time propagation path, and interaction with the main fracture at the micrometer scale. Especially during the transient process of fracture dynamic propagation, traditional sensors often have insufficient response speed or limited deployment numbers, resulting in insufficient continuous tracking and detailed characterization of fracture evolution. Summary of the Invention

[0003] To address the shortcomings of existing technologies in terms of dynamic monitoring, spatial resolution, and data acquisition during crack propagation, this specification provides a method and device for dynamic crack propagation monitoring based on distributed optical fibers. This method can simulate the hydraulically induced process of complex stress fields coupled with fluid pressure indoors, and utilize high-precision distributed optical fiber sensing to acquire strain distribution evolution data in real time during secondary crack propagation, accurately identifying the crack initiation point, propagation path, and fluid-structure interaction characteristics.

[0004] The specific technical solutions of the embodiments in this specification are as follows:

[0005] On one hand, embodiments of this specification provide a method for monitoring the dynamic propagation of cracks based on distributed optical fibers, the method comprising:

[0006] The optical fiber is pre-wound spirally around the outer surface of a cylindrical rock core at a predetermined tilt angle to establish the transformation relationship between the length position of the optical fiber and the bedding position of the rock core;

[0007] During the fracturing process of the core, the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core are obtained at each time point of the fracturing process.

[0008] Calculate the flow rate increment and the strain increment at each of the specified length positions on the optical fiber at each time point;

[0009] Based on the flow rate increment at each time point and the strain increment at each length position, the time point of crack initiation and the length position of crack initiation are determined.

[0010] The transformation relationship determines the bedding position of the crack initiation corresponding to the crack initiation length position, so that the staff can formulate a fracturing plan based on the crack initiation time point, the bedding position of the crack initiation, and the fracturing parameters of the fracturing process.

[0011] Furthermore, calculating the strain increment at each of the specified length positions on the optical fiber corresponding to the specified time point further includes:

[0012] The strain increment at length position x corresponding to time point t is calculated according to the formula Δε(x,t)=ε(x,t)-ε(x,t-1), where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t, and ε(x,t-1) represents the strain at length position x on the optical fiber corresponding to time point t-1.

[0013] Furthermore, determining the crack initiation time point and the crack initiation length position based on the flow rate increment at each time point and the strain increment at each of the aforementioned length positions further includes:

[0014] According to the formula I(x,t)=Δε(x,t) / Δε c The decision index is calculated as +ΔQ(t) / ΔQ, where I(x,t) represents the decision index at length position x on the optical fiber corresponding to time point t, and Δε(x,t) represents the strain increment at length position x on the optical fiber corresponding to time point t. c ΔQ(t) represents the strain surge threshold, and ΔQ(t) represents the flow rate increment at time point t.

[0015] Determine whether the determination index exceeds the threshold. If so, the time point corresponding to the determination index is the time point when the crack starts to crack, and the length position corresponding to the determination index is the length position when the crack starts to crack.

[0016] Furthermore, calculating the strain increment at each of the specified length positions on the optical fiber corresponding to the specified time point further includes:

[0017] According to the formula Δε(x,t)=ε(x,t)-ε base (x) Calculate the strain increment at length position x corresponding to time point t, where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t. base (x) represents the strain baseline at length position x on the optical fiber.

[0018] Furthermore, determining the crack initiation time point and the crack initiation length position based on the flow rate increment at each time point and the strain increment at each of the aforementioned length positions further includes:

[0019] Based on the strain increment at each of the length positions corresponding to each time point, construct the strain increment curve Δε(x,t);

[0020] Extract multiple continuous regions in the strain increment curve Δε(x,t) where the strain increment is greater than the noise threshold;

[0021] From the plurality of continuous regions, regions in which all strain increments are positive and the peak value exceeds a predetermined threshold Δε(th) are identified to obtain at least one strain hotspot region;

[0022] The time point corresponding to the position of the largest strain increment within the strain hotspot region is determined as the target time point;

[0023] Determine whether the traffic increase corresponding to the target time point exceeds the traffic surge threshold;

[0024] If so, the length position with the largest strain increment within the strain hotspot area is taken as the crack initiation length position, and the target time point is taken as the crack initiation time point.

[0025] Furthermore, calculating the traffic increment corresponding to the aforementioned time point further includes:

[0026] The flow increment at time point t is calculated using the formula ΔQ(t) = Q(t) - Q(t-1), where Q(t) represents the flow at time point t and Q(t-1) represents the flow at time point t-1.

[0027] Furthermore, after determining the time point of crack initiation and the length and location of crack initiation, the method further includes:

[0028] Multiple time points after the crack initiation time are used as the time points to be analyzed.

[0029] The length positions before and after the crack initiation length position are taken as the length positions to be analyzed;

[0030] Determine the strain increment at each analysis length position on the optical fiber corresponding to the analysis time point;

[0031] Based on the strain increment at each length position on the optical fiber corresponding to the time point to be analyzed, it is determined whether the crack at the bedding location where the crack initiation occurs propagation.

[0032] Furthermore, the method also includes:

[0033] If the crack at the bedding location where the crack initiation occurs propagates, it is determined whether the strain at each length position to be analyzed on the optical fiber is stable. If it is stable, the time point when the strain at each length position to be analyzed on the optical fiber reaches stability is determined. The crack propagation time is calculated based on the time point when the strain reaches stability and the time point when the crack initiation occurs, so that the staff can formulate the fracturing plan based on the time point when the crack initiation occurs, the bedding location where the crack initiation occurs, the crack propagation time, and the fracturing parameters of the fracturing process.

[0034] Furthermore, constructing the transformation relationship between the length position of the optical fiber and the bedding position of the rock core further includes:

[0035] Using the starting end of the core as a reference zero point, determine the fiber length coordinates corresponding to the reference zero point;

[0036] The position on the rock core at a specified distance from the reference zero point is used as the calibration point, and the fiber length coordinates corresponding to the calibration point are determined.

[0037] The scaling factor is calculated based on the fiber length coordinates corresponding to the reference zero point, the fiber length coordinates corresponding to the calibration point, and the specified value. The scaling factor is the transformation relationship.

[0038] On the other hand, embodiments of this specification also provide a crack dynamic propagation monitoring device based on distributed optical fiber, the device comprising:

[0039] The transformation relationship determination unit is used to construct the transformation relationship between the length position of the optical fiber and the bedding position of the rock core after the optical fiber is spirally wound around the outer surface of the cylindrical rock core at a predetermined tilt angle.

[0040] The data acquisition unit is used to acquire the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core at each time point during the fracturing process of the core.

[0041] An incremental calculation unit is used to calculate the flow increment and the strain increment at each length position on the optical fiber at each time point;

[0042] The crack initiation analysis unit is used to determine the crack initiation time point and the crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position.

[0043] The conversion unit is used to determine the bedding position of the crack initiation corresponding to the length position of the crack initiation according to the transformation relationship, so that the staff can formulate a fracturing plan based on the time point of crack initiation, the bedding position of crack initiation, and the fracturing parameters of the fracturing process.

[0044] Using the embodiments in this specification, millimeter-level dynamic sensing of axial-circumferential multi-directional strain in rock cores is achieved through a spirally wound fiber arrangement. Based on the flow rate increment at each time point during the fracturing process and the strain increment at each length position on the optical fiber, the time point of fracture initiation and the length position of fracture initiation are determined. Then, the length position of fracture initiation is converted to obtain the bedding position of fracture initiation on the rock core. It has high measurement accuracy, flexible deployment, and good repeatability, overcoming the shortcomings of insufficient resolution in traditional acoustic emission and strain gauge monitoring, and has important engineering application value. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this specification or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the embodiments of this specification. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 The diagram shown is a flowchart illustrating a crack dynamic propagation monitoring method based on distributed optical fiber in an embodiment of this specification.

[0047] Figure 2 The diagram shown is a structural schematic of the fracturing system in an embodiment of this specification;

[0048] Figure 3 The figure shown is a three-dimensional cloud diagram of the strain data changing over time in an embodiment of this specification;

[0049] Figure 4 The diagram shown is a flowchart illustrating the process of determining whether a crack is expanding in an embodiment of this specification.

[0050] Figure 5 The figure shown is a pressure-flow curve obtained from the dynamic propagation monitoring of vertical bedding in a layered shale as described in this specification.

[0051] Figure 6 The diagram shown is a structural schematic of a crack dynamic propagation monitoring device based on distributed optical fiber in an embodiment of this specification.

[0052] Figure 7 The diagram shown is a structural schematic of the computer device in an embodiment of this specification.

[0053] [Explanation of Figure Markers]:

[0054] 601. Unit for determining transformation relationships;

[0055] 602. Data Acquisition Unit;

[0056] 603. Incremental Calculation Unit;

[0057] 604. Crack Initiation Analysis Unit;

[0058] 605. Conversion Unit;

[0059] 702. Computer equipment;

[0060] 704. Processing equipment;

[0061] 706. Storage resources;

[0062] 708. Drive system;

[0063] 710. Input / Output Module;

[0064] 712. Input devices;

[0065] 714. Output devices;

[0066] 716. Presentation equipment;

[0067] 718. Graphical User Interface;

[0068] 720. Network interface;

[0069] 722. Communication link;

[0070] 724. Communication bus. Detailed Implementation

[0071] The technical solutions in the embodiments of this specification will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the embodiments of this specification, and not all embodiments. Based on the embodiments of this specification, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the embodiments of this specification.

[0072] It should be noted that the terms "first," "second," etc., in the description, claims, and accompanying drawings of the embodiments herein are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the embodiments described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, apparatus, product, or device that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or devices.

[0073] It should be noted that the acquisition, storage, use, and processing of data in the technical solutions of the embodiments of this specification all comply with the relevant provisions of national laws and regulations.

[0074] It should be noted that in the embodiments of this specification, certain software, components, models and other existing solutions in the industry may be mentioned. These should be regarded as exemplary and are only intended to illustrate the feasibility of implementing the technical solution of this application. However, they do not mean that the applicant has used or necessarily used the solution.

[0075] To address the problems existing in the prior art, this specification provides a method for monitoring the dynamic propagation of cracks based on distributed optical fibers. Figure 1 The diagram illustrates a flow chart of a crack dynamic propagation monitoring method based on distributed optical fibers, as described in this specification. The diagram depicts the process of monitoring the dynamic propagation of cracks in a rock core by measuring the strain of the optical fiber. The order of steps listed in the embodiment is merely one possible execution order among many and does not represent the only possible order. In actual system or device products, the methods shown in the embodiment or the accompanying drawings can be executed sequentially or in parallel. Specifically, as shown... Figure 1 As shown, this method can be performed by a computer, and the method may include:

[0076] Step 101: The optical fiber is spirally wound around the outer surface of the cylindrical rock core at a predetermined tilt angle to establish the transformation relationship between the length position of the optical fiber and the bedding position of the rock core;

[0077] Step 102: During the fracturing process of the core, obtain the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core at each time point of the fracturing process.

[0078] Step 103: Calculate the flow rate increment and strain increment at each of the specified length positions on the optical fiber at each time point;

[0079] Step 104: Based on the flow rate increment at each time point and the strain increment at each length position, determine the crack initiation time point and the crack initiation length position.

[0080] Step 105: Determine the bedding position of the crack initiation corresponding to the length position of the crack initiation based on the transformation relationship, so that the staff can formulate a fracturing plan based on the time point of crack initiation, the bedding position of crack initiation, and the fracturing parameters of the fracturing process.

[0081] In the embodiments described in this specification, a bedding shale core sample from the target shale layer is first selected. The core sample is cylindrical in shape, with the bedding planes roughly parallel to the bottom surface of the cylinder. The two ends of the core sample are ground smooth, and the bedding orientation is marked on the sidewalls.

[0082] Then, the optical fiber is spirally wound around the outer surface of the core at a predetermined tilt angle. Depending on the needs, the optical fiber can be either attached to the core surface or embedded in a pre-fabricated groove in the core. In a preferred embodiment, a spiral attachment method is used: the bare optical fiber is tightly wound around the surface of the core column at a certain spiral angle (e.g., approximately 20°), one or more turns, ensuring full contact between the optical fiber and the core surface.

[0083] It can be understood that, in the embodiments of this specification, the fiber optic sensor is preferably spirally wound and attached to the core surface at a predetermined tilt angle, rather than being attached along the axial direction of the core. Spiral winding at a predetermined tilt angle has the following technical advantages:

[0084] (1) Stronger adhesion and strain transfer: The spiral path allows the optical fiber to be repeatedly attached along the circumference of the core, resulting in more uniform stress and avoiding the local peeling or uneven strain problems that may occur when attaching the optical fiber along the axial direction of the core. If it is embedded in a pre-cut groove in the core, the spiral winding can be tightly embedded in the pre-cut shallow groove, forming a good bonding state, which is not easy to loosen and fall off, thus ensuring that the strain of the optical fiber can be reliably transmitted to reflect the deformation of the core surface. In addition, compared with the straight-line layout scheme, the spiral layout is conducive to preventing the easy detachment and breakage of the optical fiber sensor, which significantly improves the stability and durability of the sensing system.

[0085] (2) Improved Spatial Resolution: Helical winding allows for a longer fiber optic sensing path over a limited core length. The fiber optic cable winds around the core with a fixed pitch, covering it multiple times. This means that for every small distance the fiber optic cable travels along the core length, a certain length of sensitive fiber is accumulated. When using distributed fiber optic sensing (such as Brillouin or Rayleigh scattering), the minimum measurement interval for the fiber length is fixed; helical winding increases the fiber length contained per unit core length, thus amplifying the spatial sampling density. In other words, the helical path effectively improves the intrinsic spatial resolution of the fiber optic sensor, enabling it to capture finer strain distribution changes. This is particularly advantageous for detecting the initiation signs of fine cracks.

[0086] (3) All-round strain sensing: The spiral winding covers all circumferential angles of the core. Compared with the straight winding which is only laid out along a single generatrix, the spiral method can sense the comprehensive strain of the core in both the circumferential and axial directions. When the core deforms or cracks propagate, the strain components in different directions will be coupled and reflected on the spiral optical fiber, making the sensing signal more comprehensive. In particular, when the cracks start on one side of the core, the spiral-wound optical fiber can more sensitively capture the strain anomaly because it bypasses that area. Therefore, the spiral winding method has significant technical advantages in improving sensing sensitivity and ensuring data integrity, and overall improves the reliability and accuracy of core strain monitoring.

[0087] To prevent optical fiber slippage and liquid corrosion, an adhesive (such as acetone) can be coated on the fiber surface for fixation, and then covered with an elastic, waterproof material (such as silicone rubber) to form a protective layer. If strain data within the core is required, a narrow groove can be cut along the length of the core, the optical fiber can be embedded within it, and the groove can be sealed with adhesive to ensure close contact between the fiber and the rock. The optical fiber can be folded back at one end of the core to form a U-shape and then return along another path, thus enabling monitoring of multiple survey lines on a single fiber and improving the efficiency of acquiring spatially distributed data.

[0088] If the rock core is long, the optical fiber can be wound around its outer surface using a multi-turn spiral winding method. In other words, the optical fiber can be wound around the rock core multiple times until it covers the entire length. Multi-turn spiral winding is essentially equivalent to extending the path of the sensing optical fiber on the rock core, which has a positive impact on monitoring accuracy and spatial resolution.

[0089] (1) Improved monitoring accuracy: Multi-turn winding means that each location in the core may be traversed by the optical fiber multiple times or for longer periods, thereby increasing sensing redundancy and signal strength. When strain changes occur, the optical fiber will sense strain several times in adjacent turns, making the measurement more robust. This helps to improve the signal-to-noise ratio and accuracy of strain measurement and reduce the impact of single-point measurement errors. At the same time, multi-turn winding allows the same crack event to generate responses at multiple locations on the optical fiber, and comparison can improve the reliability of the interpretation.

[0090] (2) Improved spatial resolution: As mentioned above, the spatial resolution of distributed optical fibers depends on the sampling interval along the fiber length. Multiple turns of the spiral are equivalent to reducing the axial pitch of the helix, resulting in a greater fiber length corresponding to a unit core length. This reduces the distance between adjacent fiber sampling points along the core axis, lowering the minimum resolvable spatial scale, thus improving spatial resolution. For example, where a 1 cm fiber along a straight line originally corresponds to a 1 cm core length, if the fiber is wound to create three times the length, then each 1 cm of core contains approximately 3 cm of fiber. The instrument's original 1 cm reading interval now corresponds to a core length resolution of approximately 0.33 cm. Therefore, multiple turns of the spiral can detect finer strain gradients and local changes, which is highly advantageous for capturing the initial location of crack initiation and identifying subtle slippage at bedding planes.

[0091] (3) Expanded monitoring range and integrity: When the core sample is long, a single spiral may not be able to cover the entire length. However, by continuously winding multiple turns, it can be ensured that the optical fiber sensor covers the entire area from the beginning to the end of the core sample, leaving no blind spots. This is especially important for the fracturing of long core samples, ensuring that any abnormal strain occurring at any location can be detected in a timely manner. It should be noted that when winding multiple turns, it is important to ensure that the optical fibers do not interfere with each other and maintain good fit, so as not to affect signal interpretation. In general, as long as the installation and calibration are proper, increasing the number of winding turns will significantly improve the precision and reliability of strain monitoring.

[0092] In the embodiments of this specification, the structural schematic diagram of the fracturing system can be as follows: Figure 2 As shown in the embodiment of this specification, a detachable fiber optic core holder is designed to fix the core and adapt to the deployment requirements of fiber optic sensing. The holder consists of end caps (one for injection and the other for exit), a pressure-sealing rubber sleeve, fiber optic channels, and O-rings. The end caps adopt an integrated two-in-one structure, with radially distributed fiber optic channels located near the sidewall of the core column. The core is placed inside the central rubber sleeve of the holder, which provides radial pressure sealing. After the fiber optic cable is laid along the core surface, its two ends are led out of the holder from the fiber optic channels of the upper and lower end caps. Under pressure, the inner wall of the rubber sleeve fits tightly against the fiber optic channels on the end caps, effectively sealing and protecting the fiber optic cable. This holder allows for easy disassembly and replacement of the core sample and fiber optic cable. When the fiber optic cable or core sample is damaged during fracturing, it can be quickly replaced without replacing the entire device. The clamp can accommodate standard core column samples (50 mm in length and 25 mm in diameter) and can withstand confining pressures of up to 50 MPa and internal injection pressures of at least 30 MPa. It is suitable for core loading and dynamic fracture propagation monitoring under simulated high-pressure conditions in deep reservoirs.

[0093] After the optical fiber is laid on the core, both ends of the fiber are led out through the fiber optic channels reserved in the clamp plugs and connected to the optical fiber demodulation equipment. It should be noted that after the optical fiber is laid and the equipment is connected, calibration methods (such as applying instantaneous temperature shock or strain disturbance to the optical fiber at a specific location on the core surface) can be used to mark the physical location corresponding to the measured length of the optical fiber, so as to accurately locate the crack location during subsequent data analysis.

[0094] This specification describes an embodiment that uses a dynamic distributed optical fiber sensing system (OSI-D) based on optical frequency domain reflectance (OFDR) technology to monitor the temperature of the entire fracturing process in real time. The OSI-D measurement point spatial resolution can reach up to 0.64 mm, with a temperature measurement accuracy of ±0.1℃ and a strain measurement accuracy of ±1 με. The sampling rate is 120 Hz, and through optimized algorithms, real-time dynamic demodulation of the OFDR signal can be achieved. OFDR is a coherent detection technology based on continuous frequency modulation of light. It locates the scattered signal by measuring the frequency of the Rayleigh scattering signal generated by the modulated probe light, achieving millimeter-level spatial resolution and extremely high sensing accuracy. Specifically, the measurement process involves a linearly swept beam emitted by a tunable laser, which is split into two paths by a coupler. One path enters a reference fiber and is reflected back to the coupler by a tail mirror as the local oscillator reference light. The other path enters the measurement fiber to sense the temperature / strain changes acting on the fiber. After measurement, Rayleigh scattering light is continuously generated within the fiber and returns along the original path. After the two backscattered beams undergo beat frequency interference in the coupler, they enter the photodetector, which converts the optical signal into an electrical signal, thereby obtaining the Rayleigh scattering distribution information along the entire optical fiber. By demodulating the Rayleigh scattering spectrum signal, strain and temperature data can be obtained.

[0095] The fracturing method described in this specification is used to simulate the hydraulic fracturing process of layered shale under complex stress conditions. It includes the following steps: First, rock sample preparation is performed. Shale core samples with obvious layering structures are selected (e.g., columnar samples taken along the bedding planes). The sample size is ensured to fit the holder, and both end faces are machined flat to ensure good fit with the plugs. Then, the aforementioned optical fibers are laid on the core in a selected manner, and the holder is assembled and sealed: the core with the optical fibers wound around it is placed in the center of the rubber sleeve, aligned with the upper and lower plugs, and the fiber lead ends are fused to the fiber leads on the plug flange using an optical fiber fusion splicer, ensuring a strong connection and that the optical signal loss is within acceptable limits. Next, the front and rear plugs are fixedly connected to the rubber sleeve, the holder is installed in the confining pressure cavity (such as a steel pressure vessel), and the confining pressure system is connected. The confining pressure system is activated to apply confining pressure to a predetermined value (e.g., simulating formation confining pressure of 20 MPa or higher). A medium (such as water or oil) is injected into the holder's rubber sleeve using a manual hydraulic pump to pressurize the core and achieve confining pressure loading. After applying confining pressure, maintain the pressure for a period of time (e.g., 30 minutes) to ensure the rock sample's pressure balance stabilizes, while observing the fiber optic strain monitoring signal as it stabilizes. Then connect the fluid injection system, with the injection channel at the center of the injection end plug connecting the high-pressure injection pump and the storage tank. Slowly increase the injection pressure or control the injection flow rate to begin injecting fracturing fluid (e.g., a water-based liquid of suitable viscosity, which can be mixed with tracer dye for easier observation) into the core. During fracturing, you can choose between a constant-rate injection mode (constant flow rate pumping) or a constant-pressure injection mode (using servo control to maintain the injection pressure at the target value): when using constant-rate injection, the pump pressure changes with the rock fracture behavior; when using constant-pressure injection, the flow rate shows a sharp increase as the fracture expands.

[0096] The embodiments in this specification preferably employ constant pressure injection to simulate the constant pressure injection conditions in a field fracturing wellbore. In this case, a predetermined injection pressure is maintained during the initial stage of fracturing, and the pressure and flow rate responses are monitored. As the injection process progresses, when cracks gradually form and propagate within the core under the injection pressure, the main fracture will form instantaneously after overcoming the formation's tensile strength. This is manifested as an initial sharp drop in the pressure curve (under constant rate injection) or a sharp increase in the flow rate curve (under constant pressure injection). Figure 5 As shown. After the main fracture initiates, the injection process continues. During this time, secondary fractures in the rock sample are successively activated along the bedding planes, manifested as a series of small fluctuations in the pressure signal, while the flow rate signal shows a step-like upward trend. Throughout the fracturing process, a distributed fiber optic sensing system collects strain changes at various locations in the core in real time, while simultaneously recording injection pressure and flow rate data, until the set injection time or volume is reached, or the rock sample exhibits a penetrating fracture that prevents further pressurization. Injection is then stopped, the confining pressure is slowly released, and the rock sample is removed from the holder.

[0097] It should be noted that, Figure 2The fracturing system shown is merely exemplary, and the crack dynamic propagation monitoring method based on distributed optical fiber in the embodiments of this specification is also applicable to other fracturing systems.

[0098] The embodiments in this specification require establishing the transformation relationship between the length position of the optical fiber and the bedding position of the rock core, which can be achieved through the following steps:

[0099] The starting end of the core (i.e., one end face of the core) is used as the reference zero point to determine the fiber length coordinate L0 corresponding to the reference zero point;

[0100] The position on the core that is at a distance Z0 from the reference zero point is used as the calibration point, and the fiber length coordinate L1 corresponding to the calibration point is determined.

[0101] The scaling factor is calculated based on the fiber length coordinate L0 corresponding to the reference zero point, the fiber length coordinate L1 corresponding to the calibration point, and the specified value Z0. The scaling factor is the transformation relationship.

[0102] Specifically, the proportionality coefficient k can be expressed as: k=(L1-L0) / Z0, and based on this, the optical fiber length coordinate L(z) corresponding to any position z on the core is L0+k×z.

[0103] Similarly, the length readings of any fiber optic monitoring point can be substituted into the calculation of the actual core position. For higher accuracy, multiple calibration points can be added for segmented calibration to correct for minor nonlinear deviations that may be introduced by the helical path. Various methods can be employed during calibration to ensure accurate correspondence. For example, a transient signal can be applied to the fiber optic cable at a known location on the core to generate a recognizable response (e.g., using liquid nitrogen to induce a sudden local temperature change at one end of the core, forming a significant characteristic signal in the fiber optic measurement curve), thereby accurately determining the starting point of the fiber optic measurement and the positions of each calibration point. This pre-calibration method allows subsequent steps to directly locate the bedding positions on the core based on the fiber optic length, ensuring accurate correlation of crack dynamic propagation monitoring.

[0104] In this embodiment of the specification, during the core fracturing process, the strain at each length position on the optical fiber corresponding to each time point of the fracturing process and the flow rate of the fluid injected into the core are obtained. Then, the flow rate increment and the strain increment at each length position on the optical fiber corresponding to each time point are calculated respectively. Based on the flow rate increment at each time point and the strain increment at each length position, the time point of fracture initiation and the length position of fracture initiation are determined.

[0105] If no new fractures have yet appeared inside the core, the injection flow rate will remain at a low level while the pressure remains relatively stable (in constant pressure mode, the control system adjusts the flow rate to maintain the pressure). As time progresses, when the weak bedding planes of the core begin to be compressed open, fiber optic strain monitoring will first detect local strain anomalies, such as a rapid increase in tensile strain on a certain section of fiber, indicating fracture initiation. Subsequently, if the main fracture breaks through, the injection pressure will suddenly drop in constant rate mode, while a sudden increase in flow rate will occur in constant pressure mode. Under the constant pressure injection conditions of this embodiment, when a significant jump in the instantaneous flowmeter reading is observed and maintained at a high level, and at the same time, the fiber optic strain data shows a drastic change at a certain bedding location, it can be determined that the main fracture has formed and is expanding. Continue constant pressure injection to enter the fracture expansion stage. Since the formation of the main fracture provides the main channel for fluid to enter the rock sample, the fluid will then permeate and diffuse along other weak planes of the rock under confining pressure, inducing the formation of new secondary fractures. Whenever a new lamination crack is excited, a new strain peak or abrupt signal will be detected at the corresponding location of the optical fiber, and the pressure sensor may also record subtle pulse fluctuations, while the flow rate gradually increases to a higher plateau.

[0106] During constant pressure injection, this specification's embodiments further creatively introduce a quantitative judgment method based on fiber optic strain and flow rate signals to identify whether new bedding fractures have been activated. Because when the injection pressure is kept constant, once a new fracture is activated, the pressure drop of the system remains unchanged, but the number of channels available for fluid entry suddenly increases, often leading to a jump in injection flow rate, accompanied by a sudden change in local core strain. Based on this principle, the following judgment model can be established: when the fiber optic strain signal and flow rate signal satisfy the synchronous abrupt change condition, it is determined that a new bedding fracture has initiated.

[0107] Specifically, the strain surge threshold Δε of the optical fiber strain is set. c (This can be determined based on background noise and historical data) and the flow surge threshold ΔQ. Under constant pressure injection conditions, if at time point t, the strain increment Δε(x,t) = ε(x,t) - ε(x,t-1) at a certain monitoring location of the optical fiber is greater than the threshold Δε c If, at the same time point t, the injection flow rate shows a significant increase relative to the stable value (ΔQ(t) > ΔQ), then it can be determined that the new bedding fracture at the corresponding location x in the core has been activated (initiated) and has begun to propagate. In short, when the pressure is kept constant, "sudden increase in local strain + sudden increase in flow rate" is considered the combination of criteria for fracture initiation.

[0108] In the embodiments of this specification, the traffic increment corresponding to a point in time refers to the increase in traffic at that point in time relative to the previous point in time. Specifically, calculating the traffic increment corresponding to the point in time further includes:

[0109] The flow increment at time point t is calculated using the formula ΔQ(t) = Q(t) - Q(t-1), where Q(t) represents the flow at time point t and Q(t-1) represents the flow at time point t-1.

[0110] According to one embodiment of this specification, in order to more accurately determine whether a crack has initiated, this embodiment of the specification determines the crack initiation time point and the crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each of the stated length positions, which may include the following steps:

[0111] According to the formula I(x,t)=Δε(x,t) / Δε c The decision index is calculated as +ΔQ(t) / ΔQ, where I(x,t) represents the decision index at length position x on the optical fiber corresponding to time point t, and Δε(x,t) represents the strain increment at length position x on the optical fiber corresponding to time point t. c ΔQ(t) represents the strain surge threshold, and ΔQ(t) represents the flow rate increment at time point t.

[0112] Determine whether the determination index exceeds the threshold. If so, the time point corresponding to the determination index is the time point when the crack starts to crack, and the length position corresponding to the determination index is the length position when the crack starts to crack.

[0113] In the embodiments of this specification, the threshold should be greater than or equal to 2. The specific value can be set according to actual needs, and the embodiments of this specification do not impose any restrictions.

[0114] In addition, small fluctuations in the pressure signal can be considered: under constant pressure control conditions, instantaneous pressure disturbances may occur when a new crack initiates. At this time, the pressure holding system will feed back the pressure fluctuation signal according to the amplitude of the pressure fluctuation. If the judgment index exceeds the threshold and the pressure fluctuation amplitude at the time point corresponding to the judgment index exceeds the threshold value, it indicates that the crack has initiated.

[0115] In another embodiment of this specification, a waterfall plot or a three-dimensional contour plot can also be drawn to visualize the dynamic process of crack propagation by showing the strain data changing over time. For example, a three-dimensional contour plot of strain data changing over time can be as follows: Figure 3 As shown in the figure, analyzing this figure can determine the time and location of the initial appearance of the crack: for example, at the beginning of a certain time frame, a significant strain hot spot area appears at a certain bedding plane in the middle of the rock core, which is judged as the crack initiation point.

[0116] Specifically, crack initiation behavior can be determined by observing the changes in strain over time and space. The basic logic is that crack initiation leaves characteristic signals in the strain distribution of the optical fiber, manifested as a localized, rapid strain change (usually an increase in tensile strain) at a certain location, forming a strain "hotspot." The appearance and expansion of this hotspot region can be used to identify the time and location of crack initiation.

[0117] In practice, the following steps can be taken:

[0118] (1) Baseline establishment: The data from the initial stage of fracturing injection were selected as the strain baseline ε. base (x).

[0119] (2) Incremental calculation: During the fracturing process, according to the formula Δε(x,t)=ε(x,t)-ε base (x) Calculate the strain increment at length position x corresponding to time point t, where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t. base (x) represents the strain baseline at length position x on the optical fiber.

[0120] (3) Hot spot determination: Search for continuous regions in the Δε(x,t) curve that are greater than the noise threshold. If there is a continuous interval in which the strain increment is significantly positive and the peak value exceeds the predetermined threshold Δε(th), then it is determined to be a strain hot spot region.

[0121] (4) Location of crack initiation point: Select the point x with the largest strain increment within the strain hotspot area. max The location of the crack initiation point is used to identify the location, and its occurrence time t is recorded. max As the target time point. Determine the target time point t. max Does the corresponding traffic increase exceed the traffic surge threshold? If so, then select the point x with the largest strain increase within the strain hotspot area. max The target time point t is used as the length position of the crack initiation point. max This refers to the time point at which the crack begins to crack.

[0122] Alternatively, the peak value can be precisely located by detecting the change in the spatial derivative of strain along the fiber length: for example, finding the point x where dε / dx changes from positive to negative (a local maximum occurs) and the strain ε(x) exceeds a threshold. max The condition is satisfied: there exists a parameter δ>0, when |xx max When |<δ:

[0123] ;

[0124] x that satisfies the above conditions max The corresponding strain peak point is used to determine the length of the crack initiation point.

[0125] Then, based on the transformation relationship, the bedding location corresponding to the crack initiation length position is determined.

[0126] As time progresses, if the strain in this region continues to increase and expands to both sides, it verifies that this indicates the crack is extending outward from its initiation point. When the fiber strain exhibits a "wedge-shaped" characteristic along its length, with tensile strain concentrated in the middle and compressive strain bands on both sides, it indicates that a layered crack parallel to that fiber location has initiated and is expanding towards both ends. Based on this, the embodiments in this specification can incorporate pattern recognition into the algorithm to automatically analyze fiber strain waterfall plots or time series: once the aforementioned "tensile strain peak region" characteristic pattern is detected, the system can automatically determine the crack initiation and output the initiation location and time.

[0127] Specifically, such as Figure 4 As shown, the steps to determine whether a crack is propagating include:

[0128] Step 401: Select multiple time points after the crack initiation time as the time points to be analyzed;

[0129] Step 402: Select multiple length positions before and after the crack initiation length position as the length positions to be analyzed;

[0130] Step 403: Determine the strain increment at each length position on the optical fiber corresponding to the time point to be analyzed;

[0131] Step 404: Based on the strain increment at each length position on the optical fiber corresponding to the time point to be analyzed, determine whether the crack at the bedding location where the crack initiation occurs has propagated.

[0132] In the embodiments of this specification, if the strain increment at each position along the length to be analyzed on the optical fiber corresponding to the time point to be analyzed exceeds the strain surge threshold Δε c This indicates that the crack has expanded.

[0133] Furthermore, if the crack extends, the duration of crack extension can be calculated so that the personnel can formulate the fracturing plan based on the crack initiation time, the bedding location of the crack initiation, the duration of crack extension, and the fracturing parameters of the fracturing process.

[0134] Specifically, if the crack at the lamellae location where the crack initiation occurs expands, it is determined whether the strain at each length position to be analyzed on the optical fiber is stable. If it is stable, the time point when the strain at each length position to be analyzed on the optical fiber reaches stability is determined, and the crack propagation time is calculated based on the time point when the strain reaches stability and the time point when the crack initiation occurs.

[0135] It is feasible to determine whether the strain fluctuation within a specified time range exceeds a threshold. If it does not exceed the threshold, it indicates that the strain has reached stability and the crack no longer propagates. The last time point corresponding to this time range is taken as the time point when the strain reaches stability.

[0136] Through this logical method and criterion formula, the identification of crack initiation points is automated and quantified, greatly improving the intelligence level of monitoring and analysis, and enabling timely and accurate extraction of key information on crack initiation.

[0137] Based on the same inventive concept, embodiments of this specification also provide a crack dynamic propagation monitoring device based on distributed optical fiber, such as... Figure 6 As shown, it includes:

[0138] The transformation relationship determination unit 601 is used to construct the transformation relationship between the length position of the optical fiber and the bedding position of the rock core after the optical fiber is spirally wound around the outer surface of the cylindrical rock core at a predetermined tilt angle.

[0139] The data acquisition unit 602 is used to acquire the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core at each time point during the fracturing process of the core.

[0140] The incremental calculation unit 603 is used to calculate the flow increment corresponding to each time point and the strain increment at each length position on the optical fiber;

[0141] The crack initiation analysis unit 604 is used to determine the crack initiation time point and the crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position.

[0142] The conversion unit 605 is used to determine the bedding position of the crack initiation corresponding to the length position of the crack initiation according to the conversion relationship, so that the staff can formulate a fracturing plan based on the time point of crack initiation, the bedding position of crack initiation, and the fracturing parameters of the fracturing process.

[0143] The beneficial effects obtained by the above-described device are the same as those obtained by the above-described method, and will not be described in detail in the embodiments of this specification.

[0144] like Figure 7The diagram illustrates the structure of a computer device according to an embodiment of this specification. The computer device in this embodiment is capable of executing the methods described in the embodiments of this specification. The computer device 702 may include one or more processing devices 704, such as one or more central processing units (CPUs), each of which may implement one or more hardware threads. The computer device 702 may also include any storage resource 706 for storing information of any kind, such as code, settings, data, etc. Without limitation, for example, the storage resource 706 may include any type of RAM, any type of ROM, flash memory, hard disk, optical disk, etc. More generally, any storage resource can use any technology to store information. Furthermore, any storage resource may provide volatile or non-volatile retention of information. Furthermore, any storage resource may represent a fixed or removable component of the computer device 702. In one case, when the processing device 704 executes associated instructions stored in any storage resource or combination of storage resources, the computer device 702 may perform any operation of the associated instructions. The computer device 702 also includes one or more drive systems 708 for interacting with any storage resources, such as hard disk drive systems, optical disk drive systems, etc.

[0145] Computer device 702 may also include an input / output module 710 (I / O) for receiving various inputs (via input device 712) and providing various outputs (via output device 714). A specific output mechanism may include a presentation device 716 and an associated graphical user interface (GUI) 718. In other embodiments, the input / output module 710 (I / O), input device 712, and output device 714 may be omitted, and the device may function solely as a computer device within a network. Computer device 702 may also include one or more network interfaces 720 for exchanging data with other devices via one or more communication links 722. One or more communication buses 724 couple the components described above together.

[0146] Communication link 722 can be implemented in any way, such as via a local area network, a wide area network (e.g., the Internet), a point-to-point connection, or any combination thereof. Communication link 722 may include any combination of hardwired links, wireless links, routers, gateway functions, name servers, etc., governed by any protocol or combination of protocols.

[0147] This specification also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements the above-described method.

[0148] This specification also provides computer-readable instructions, wherein when a processor executes the instructions, the program therein causes the processor to perform the above-described method.

[0149] It should be understood that in the various embodiments of this specification, the sequence number of each process does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this specification.

[0150] It should also be understood that, in the embodiments of this specification, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Furthermore, in the embodiments of this specification, the character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0151] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed in this specification can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of each example have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of the embodiments in this specification.

[0152] Those skilled in the art will clearly understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0153] In the embodiments provided in this specification, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the couplings or direct couplings or communication connections shown or discussed may be indirect couplings or communication connections through some interfaces, devices, or units, or they may be electrical, mechanical, or other forms of connection.

[0154] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of the embodiments described in this specification, depending on actual needs.

[0155] Furthermore, the functional units in the various embodiments of this specification can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0156] If the integrated unit is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solutions of the embodiments of this specification, in essence, or the parts that contribute to the prior art, or all or part of the technical solutions, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this specification. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0157] This specification describes the principles and implementation methods of the embodiments using specific examples. The above descriptions of the embodiments are only for the purpose of helping to understand the methods and core ideas of the embodiments in this specification. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the embodiments in this specification. Therefore, the content of this specification should not be construed as a limitation on the embodiments in this specification.

Claims

1. A method for monitoring the dynamic propagation of cracks based on distributed optical fibers, characterized in that, The method includes: The optical fiber is pre-wound spirally around the outer surface of a cylindrical rock core at a predetermined tilt angle to establish the transformation relationship between the length position of the optical fiber and the bedding position of the rock core; During the fracturing process of the core, the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core are obtained at each time point of the fracturing process. Calculate the flow rate increment and the strain increment at each of the specified length positions on the optical fiber at each time point; Based on the flow rate increment at each time point and the strain increment at each length position, the time point of crack initiation and the length position of crack initiation are determined. The transformation relationship determines the bedding position of the crack initiation corresponding to the length of the crack initiation, so that the staff can formulate a fracturing plan based on the time point of crack initiation, the bedding position of crack initiation, and the fracturing parameters of the fracturing process. Calculating the strain increment at each length position on the optical fiber corresponding to the time point further includes: calculating the strain increment at length position x corresponding to time point t according to the formula Δε(x,t)=ε(x,t)-ε(x,t-1), where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t, and ε(x,t-1) represents the strain at length position x on the optical fiber corresponding to time point t-1; or, According to the formula Δε(x,t)=ε(x,t)-ε base (x) Calculate the strain increment at length position x corresponding to time point t, where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t. base (x) represents the strain baseline at length position x on the optical fiber; When calculating the strain increment at length position x corresponding to time point t according to the formula Δε(x,t)=ε(x,t)-ε(x,t-1), determining the crack initiation time point and crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position further includes: According to the formula I(x,t)=Δε(x,t) / Δε c The decision index is calculated as +ΔQ(t) / ΔQ, where I(x,t) represents the decision index at length position x on the optical fiber corresponding to time point t, and Δε(x,t) represents the strain increment at length position x on the optical fiber corresponding to time point t. c Let ΔQ(t) represent the strain surge threshold, ΔQ(t) represent the flow rate increment at time point t, and ΔQ represent the flow rate surge threshold. Determine whether the determination index exceeds the threshold. If it does, the time point corresponding to the determination index is the time point at which the crack begins to crack, and the length position corresponding to the determination index is the length position at which the crack begins to crack. When according to the formula Δε(x,t)=ε(x,t)-ε base (x) When calculating the strain increment at length position x corresponding to time point t, determining the crack initiation time point and crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position further includes: Based on the strain increment at each length position corresponding to each time point, a strain increment curve Δε(x,t) is constructed; multiple continuous regions where the strain increment is greater than a noise threshold are extracted from the strain increment curve Δε(x,t); regions where the strain increment is positive and the peak value exceeds a predetermined threshold Δε(th) are found from the multiple continuous regions to obtain at least one strain hotspot region; the time point corresponding to the length position with the largest strain increment in the strain hotspot region is determined as the target time point; it is determined whether the flow increment corresponding to the target time point exceeds the flow surge threshold; if so, the length position with the largest strain increment in the strain hotspot region is taken as the length position of the crack initiation, and the target time point is taken as the time point of the crack initiation.

2. The method according to claim 1, characterized in that, Calculating the traffic increment corresponding to the time point further includes: The flow increment at time point t is calculated using the formula ΔQ(t) = Q(t) - Q(t-1), where Q(t) represents the flow at time point t and Q(t-1) represents the flow at time point t-1.

3. The method according to claim 1, characterized in that, After determining the time point of crack initiation and the length and location of crack initiation, the method further includes: Multiple time points after the crack initiation time are used as the time points to be analyzed. The length positions before and after the crack initiation length position are taken as the length positions to be analyzed; Determine the strain increment at each analysis length position on the optical fiber corresponding to the analysis time point; Based on the strain increment at each length position on the optical fiber corresponding to the time point to be analyzed, it is determined whether the crack at the bedding location where the crack initiation occurs propagation.

4. The method according to claim 3, characterized in that, The method further includes: If the crack at the bedding location where the crack initiation occurs propagates, it is determined whether the strain at each length position to be analyzed on the optical fiber is stable. If it is stable, the time point when the strain at each length position to be analyzed on the optical fiber reaches stability is determined. The crack propagation time is calculated based on the time point when the strain reaches stability and the time point when the crack initiation occurs, so that the staff can formulate the fracturing plan based on the time point when the crack initiation occurs, the bedding location where the crack initiation occurs, the crack propagation time, and the fracturing parameters of the fracturing process.

5. The method according to claim 1, characterized in that, Constructing the transformation relationship between the length position of the optical fiber and the bedding position of the rock core further includes: Using the starting end of the core as a reference zero point, determine the fiber length coordinates corresponding to the reference zero point; The position on the rock core at a specified distance from the reference zero point is used as the calibration point, and the fiber length coordinates corresponding to the calibration point are determined. The scaling factor is calculated based on the fiber length coordinates corresponding to the reference zero point, the fiber length coordinates corresponding to the calibration point, and the specified value. The scaling factor is the transformation relationship.

6. A crack dynamic propagation monitoring device based on distributed optical fiber, characterized in that, The device includes: The transformation relationship determination unit is used to construct the transformation relationship between the length position of the optical fiber and the bedding position of the rock core after the optical fiber is spirally wound around the outer surface of the cylindrical rock core at a predetermined tilt angle. The data acquisition unit is used to acquire the strain at each length position on the optical fiber and the flow rate of the fluid injected into the core at each time point during the fracturing process of the core. An incremental calculation unit is used to calculate the flow increment and the strain increment at each length position on the optical fiber at each time point; The crack initiation analysis unit is used to determine the crack initiation time point and the crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position. The conversion unit is used to determine the bedding position of the crack initiation corresponding to the length position of the crack initiation according to the transformation relationship, so that the staff can formulate a fracturing plan based on the time point of crack initiation, the bedding position of crack initiation, and the fracturing parameters of the fracturing process. Calculating the strain increment at each length position on the optical fiber corresponding to the time point further includes: calculating the strain increment at length position x corresponding to time point t according to the formula Δε(x,t)=ε(x,t)-ε(x,t-1), where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t, and ε(x,t-1) represents the strain at length position x on the optical fiber corresponding to time point t-1; or, According to the formula Δε(x,t)=ε(x,t)-ε base (x) Calculate the strain increment at length position x corresponding to time point t, where ε(x,t) represents the strain at length position x on the optical fiber corresponding to time point t. base (x) represents the strain baseline at length position x on the optical fiber; When calculating the strain increment at length position x corresponding to time point t according to the formula Δε(x,t)=ε(x,t)-ε(x,t-1), determining the crack initiation time point and crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position further includes: According to the formula I(x,t)=Δε(x,t) / Δε c The decision index is calculated as +ΔQ(t) / ΔQ, where I(x,t) represents the decision index at length position x on the optical fiber corresponding to time point t, and Δε(x,t) represents the strain increment at length position x on the optical fiber corresponding to time point t. c Let ΔQ(t) represent the strain surge threshold, ΔQ(t) represent the flow rate increment at time point t, and ΔQ represent the flow rate surge threshold. Determine whether the determination index exceeds the threshold. If it does, the time point corresponding to the determination index is the time point at which the crack begins to crack, and the length position corresponding to the determination index is the length position at which the crack begins to crack. When according to the formula Δε(x,t)=ε(x,t)-ε base (x) When calculating the strain increment at length position x corresponding to time point t, determining the crack initiation time point and crack initiation length position based on the flow rate increment corresponding to each time point and the strain increment at each length position further includes: Based on the strain increment at each length position corresponding to each time point, a strain increment curve Δε(x,t) is constructed; multiple continuous regions where the strain increment is greater than a noise threshold are extracted from the strain increment curve Δε(x,t); regions where the strain increment is positive and the peak value exceeds a predetermined threshold Δε(th) are found from the multiple continuous regions to obtain at least one strain hotspot region; the time point corresponding to the length position with the largest strain increment in the strain hotspot region is determined as the target time point; it is determined whether the flow increment corresponding to the target time point exceeds the flow surge threshold; if so, the length position with the largest strain increment in the strain hotspot region is taken as the length position of the crack initiation, and the target time point is taken as the time point of the crack initiation.

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